1. Field of the Invention
The present invention relates to a joint torque detection system, and more articularly to a joint torque detection system for detecting the joint torque exerted or imparted to a joint, for example, a robot joint.
2. Description of the Related Art
There have been proposed various joint torque detection systems for detecting the joint torque exerted or imparted to a joint, such as the one described in "Multisensory shared autonomy and tele-sensor programming--Key issues in space robotics"; pp 141-162; Robotics and autonomous Systems 11 (1993) 141-162, Elsevier. As illustrated in its FIG. 13, the detection system has a rotational gearing with inductive torque sensing for joints.
It has also been proposed to provide a six-axis force and torque sensor at the distal end of a joint such as a robot joint to detect an external force acting at a free end of the joint downstream of the sensor location so as to conduct a compliance control in order that the robot effects a smooth motion.
Since, however, the sensor, if attached at the distal end of a robot joint, can not measure an exerted force acting on a portion upstream of the sensor location, the sensor should preferably be provided in the joints to measure the torque exerting or acting on the joint, to control its motion.
When the robot joint is a robot arm which needs to work in a narrow space, the arm position should be controlled precisely such that it will not contact foreign objects such as walls. The trajectory (path) of the link distal end should accordingly be determined and controlled in an accurate manner. This is particularly significant for a robot hand, since it is an end-effector. For example, when taking an object from a narrow space or grasping an object using all of the finger links, the robot hand finger links may sometimes destroy the object or suffer from excessive load and be damaged, unless the external force (torque) is detected and a precise control is conducted based on the detected force (torque).
It is accordingly desirable to detect accurately the torque exerted or imparted to a small link mechanism such as a robot hand finger link mechanism in order that the compliance control of the link mechanism is conducted based on the detected torque.
It would be possible to affix a strain gauge on a deformable portion of a small link mechanism to determine the torque. This kind of sensor is disadvantageous in terms of service life. If a great stress acts on the deformable portion, the strain gauge will be excessively displaced and will eventually be broken. Furthermore, it becomes necessary to form such a deformable portion at the output shaft of an actuator (e.g., a DC motor) provided at the drive joint. The deformable portion is likely to project laterally, i.e., in the direction of the width of the links constituting the joint, which is disadvantageous if the links are configured to be positioned side by side like a robot hand finger link mechanism.
It would also be possible to detect the imparted torque by measuring the electric current supplied to the joint actuator. However, a link mechanism such as finger link mechanism is usually made small by increasing reduction gear ratio of a speed reducer, rendering accurate torque detection difficult due to the low efficiency of the speed reducer.